Aqueous cleaning compositions containing alkaline salts, surfactants and other adjuvants have been recently developed to clean a wide variety of surfaces. These aqueous salt cleaners are particularly advantageous since the cleaners are effective and safe to use, handle and dispose of and, accordingly, can replace the more harmful, environmentally unsafe highly basic or organic-based solvents and cleaners previously utilized. Among the particularly useful aqueous cleaners are those developed by the present assignee and which are based on alkali metal carbonates and/or bicarbonates. Separate cleaners have been developed such as for cleaning metal surfaces and, uniquely for cleaning flux residues from electronic circuit assemblies. The aqueous alkaline salt-based cleaners used for the latter purpose are marketed under the tradename ARMAKLEEN.RTM.. These cleaners are finding increasing acceptance as a replacement for the halogenated hydrocarbon and other volatile organic solvents previously used to remove flux residues, in particular, rosin flux residues. The cleanliness of electronic circuit assemblies, such as printed circuit boards or printed wiring boards, is generally regarded as being critical to their functional reliability. Ionic and nonionic contamination on circuit assemblies is believed to contribute to premature failures of the circuit assemblies by allowing short circuits to develop.
In the manufacture of electronic circuit assemblies, ionic and nonionic contamination can accumulate after one or more steps of the process. Circuit assembly materials are plated, etched, handled by operators in assembly, coated with corrosive or potentially corrosive fluxes and finally soldered.
In the fabrication of electronic circuit assemblies, soldering fluxes are first applied to the substrate board material to ensure firm, uniform bonding of the solder. These soldering fluxes fall into three broad categories: rosin fluxes, water soluble fluxes and no-clean. The rosin fluxes, which are generally only moderately corrosive and have a much longer history of use, are still widely used throughout the electronics industry. The water soluble fluxes, which are a more recent development, are being used increasingly in consumer products applications. These water soluble fluxes are very corrosive. The no-clean fluxes are a very recent development and apparently do not require removal from the circuit assemblies. Unfortunately, residues of any flux are believed to cause circuit failure if residual traces of the material are not carefully removed following soldering and, thus, remain on an electronic circuit assembly.
The water soluble flux residues have been removed with water or warm detergent solutions. Recently, the present assignee has developed an aqueous alkaline salt solution for effective removal of water soluble flux residues and which not only removes the flux residues but also has been found to remove the "white residues" which have been problematic when using water soluble fluxes. The useful cleaner for removing water soluble flux residues is described in U.S. patent application Ser. No. 08/391,015, filed, Feb. 21, 1995.
The removal of rosin flux residues from circuit assemblies, however, has been much more difficult and has traditionally been carried out with the use of chlorinated hydrocarbon solvents, mixtures of such solvents or other volatile organic solvent materials. The use of such solvents is very disadvantageous in view of the toxicity and environmental problems inherent in such materials and, accordingly, the use of such solvents has been subjected to close scrutiny by various government agencies.
To replace these solvent materials for use in cleaning rosin flux residues from circuit assemblies, the present assignee has as above said developed the ARMAKLEEN.RTM. cleaner which is an aqueous-based cleaner comprising alkali metal salts such as alkali metal carbonate and/or alkali metal bicarbonate salts, alkali metal silicates for use as a corrosion inhibitor as well as organic adjuvants such as surfactants to improve the efficacy of cleaning, anionic polymers to stabilize the silicate in solution as well as hydrotropes to maintain the surfactants in aqueous solution. This aqueous-based rosin flux remover is the subject of several U.S. patents including U.S. Pat. No. 5,234,506 which is herein incorporated by reference. These aqueous-based cleaners have been found to be very effective for removing the rosin flux residues and are substantially safer to use than the previous organic solvent-based cleaners. Unfortunately, during use, as the aqueous phase of the cleaner evaporates, there has been left behind on equipment surfaces a solids residue in the form of scale. This scale can plug drains and plague the operation of continuous cleaning equipment including conveyor belts, spray nozzles and the like, thus, not only causing a problem of aesthetics, but also reducing the efficiency of the cleaning process. The scale which remains is primarily that of carbonate, silicate and mixed salts as well as organic residues including that of the surfactants and possibly carboxylated polymer materials which are believed to render the scale intractable.
While the descaling composition of the present invention is applicable for the removal of any scale formed from inorganic salt-based aqueous cleaners, the descaler of the present invention is particularly useful in removing the scale which is formed upon use of the alkaline salt flux removing cleaners which include alkaline salts, surfactants, alkali metal silicate corrosion inhibitors and which have been found to leave a water insoluble scale upon evaporation of the aqueous phase. The unsightly scale causes maintenance along with operational problems during the cleaning process. Although the scale can be removed by use of concentrated acids or alkali materials such as sulfuric or phosphoric acid or sodium or potassium hydroxide, the use of these materials jeopardizes the health and safety of the workers and, thus, are disadvantageous. Additionally, these strong acids and bases are corrosive to the cleaning apparatus.
It would be useful to remove the scale which is produced upon the use of alkaline salt-based cleaners without using the highly corrosive concentrated acid or basic scale removers which have been previously used.
Accordingly, it is the primary objective of the present invention to provide a novel descaling formulation which will remove scale formed from inorganic salt-based cleaners.
Another object of the present invention is to provide a novel descaling composition which is at least as effective as strong acid and basic descaling compositions and which is substantially more safe to use and less toxic to the environment.
Still another object of the invention is to provide an improved method of removing from cleaning equipment water insoluble scale formed from inorganic salt-based cleaners.
These and other objects which are achieved according to the present invention can be readily discerned from the following description of the invention.